Stockbyte/Thinkstock(NEW YORK) -- The new head of the Zetas drug cartel is a former Dallas resident who is labeled as a traitor by many of his own cartel soldiers and mocked as an ex-"car washer" by his enemies. But he has risen to power thanks to a fearsome reputation for violence.

"[Miguel Angel Trevino Morales] is extremely brutal, to the point of sadism," says George Grayson, an expert on the Zetas. "He is prepared to advance his interest through unspeakable violence."

Grayson's recent book on the cartel, The Executioner's Men, opens with a scene in which Trevino Morales slowly beats a female police officer to death, in front of her colleagues, with a two-by-four.

Trevino Morales, also known as El 40 or the Monkey, became the uncontested head of the Mexico's most feared drug cartel when former kingpin Heriberto Lazcano was killed in a shootout with Mexican Marines on Sunday. Lazcano had been linked to hundreds of murders, including the massacre of 72 civilians, but Trevino Morales is allegedly even more bloodthirsty. One of his preferred methods of dealing with enemies, say authorities, is burning them alive.

Trevino Morales, 41, was born in Mexico but spent some of his formative years in Dallas, Texas, where authorities say he had a criminal record as a teenager. He has a dozen siblings and reportedly still has family in the Dallas area.

Trevino Morales joined the Zetas soon after their formation. The Zetas began in the late 1990s as the security wing of the Gulf Cartel. The 14 core members of the Zetas, including Lazcano, all had military backgrounds, and took ranks based on when they'd joined the group. Lazcano was known as Z-3. By 2004, due to the death of Z-1 and the arrest of Z-2, Lazcano had become the leader of the Zetas.

Trevino Morales, who did not have a military background, got the designation 40, with his brother taking number 42. In 2005, Trevino Morales became the boss of the Nuevo Laredo "plaza," or drug territory.

As a newly minted underboss, Trevino Morales had traditional gangster tastes for fast cars, women and fancy guns, and reportedly liked to hunt game imported from Africa. He also, however, developed a particular reputation for brutality in a group already renowned for violence. His favored methods for dispatching enemies were dismembering them while still alive, or making them into a "guiso," or stew -- stuffing them in 55-gallon oil drums, adding gasoline and burning them alive.

By 2009, Trevino Morales had been named in multiple federal indictments in Texas, Washington, D.C., and New York for alleged crimes ranging from drug trafficking, kidnapping and money laundering to ordering a half dozen murders in Laredo, Texas. The Drug Enforcement Administration offered a $5 million reward for information leading to his arrest or conviction, and accused him of controlling more than 200 operatives and smuggling hundreds of kilograms of cocaine into the U.S. weekly.

Early the next year, the Zetas finally split from the Gulf Cartel after it crossed Trevino Morales. In January 2010, the Gulf Cartel tortured and killed one of his close friends. Trevino Morales responded with an ultimatum demanding that the cartel give up the killer.

"Hand over the assassin of my friend," demanded Trevino Morales. "If you don't comply, there will be war."

The order was ignored and Trevino Morales allegedly began killing members of the Gulf Cartel en masse. The Zetas, now an independent cartel with Trevino Morales second in command, were soon battling the Gulf Cartel for control of Northern Mexico, and winning.

By 2011, however, there was a schism within the new cartel between Trevino Morales and those loyal to Lazcano. When Zetas boss "El Mamito," Enrique Rejon Aguilar, was arrested in July, he said that he had been betrayed. Though he did not name any names, the next month someone uploaded a slickly produced music video to YouTube that bluntly accused Trevino Morales of being a "Judas" who was disloyal to Lazcano and had betrayed Mamito and other Zetas to the authorities.

Addressed to all the members of "the Mafia," every major drug organization in Mexico by name, and to the general public, "The True Story of Z 40" uses a specially written "narcoballad" to detail the alleged offenses of Trevino Morales against his fellow Zetas, especially leader Lazcano.

One of the first images in the five-minute video is a picture of Judas whispering in the ear of Jesus. It then shows repeated images of Trevino Morales with the words "El Judas" under his face, and displays arrest photos of all the Zetas bosses he has allegedly betrayed, who were "captured because they trusted Z 40." Intended as a warning to Lazcano, it asks "El Lazca" why he thinks so many of his underlings have been arrested.

The video also mocks Trevino Morales as a former car washer for the Los Tejas gang, and plasters his face onto photos of police officers and a shiny-suited pop idol.

Rival groups have also disparaged Trevino Morales as a car washer. In March, Joaquin Guzman, AKA El Chapo or Shorty, the head of Mexico's other dominant drug organization, the Sinaloa Cartel, sent his men into Trevino Morales' territory to murder and dismember Zetas soldiers. He issued a public challenge to Trevino Morales on huge banners above the body parts of his victims.

One banner, accompanied by seven severed heads, accused Trevino Morales of failing to use his own head, and of being Lazcano's jockstrap.

"You will always be a car washer to me," said the banner, which was signed "El Chapo."

Another mocked Trevino Morales as a shoeshine boy, car washer and traitor who killed innocent people.

In the summer of 2012, Trevino Morales' brother Jose, a U.S. citizen, was arrested in the U.S. for moneylaundering after allegedly channeling the Zetas' drug money through a successful horseracing operation. Not long after his arrest, the split within the Zetas apparently cost 14 lives. The survivor of a mass execution in San Luis Potosi state in mid-August said that the victims and the killers were Zetas. Authorities believe the massacre was revenge by Trevino Morales on "El Taliban," a leader opposed to EL 40's ascent.

By the end of August, U.S. officials began saying that Trevino Morales seemed to have merged as the winner in the Zetas' civil war, and had officially taken operational control of the Zetas in Mexico from Lazcano.

High-ranking Zetas then began to fall. El Taliban was arrested in late September, "The Squirrel" just last Saturday. Lazcano, who was attending a baseball game with two other men, died in a firefight on Sunday.

Grayson speculates that Trevino Morales may have shared information with U.S. authorities to get better treatment for his brother Jose, who is in U.S. custody.

Trevino Morales must now direct the Zetas against the combined strength of the Gulf Cartel, the Sinaloa Cartel and other players, who have united to drive the Zetas from their "plazas." Grayson says that with Lazcano's death, El Chapo Guzman of the Sinaloa cartel will be aided in his primary goal of taking control of Nuevo Laredo, El 40's home base. Guzman has already dispatched what Grayson calls "shock troops" to help the Gulf Cartel fight the Zetas.

El Chapo's troops will be facing younger, less experienced, and less disciplined Zetas plaza bosses than in the past, says Grayson. But he also notes that the Zetas new leader, in addition to being more violent than his predecessor, may be more cautious and wily as well.